In my similar project (s3 compatible single-node storage) https://github.com/uroni/hs5 I do use proper fsync for data and metadata durability. But it can be turned of via switch. It is a pet peeve of mine that the defaults should always be to fsync. I do have a section on this in my README of the project.

I also do have an optional WAL. Maybe I should add an additional mode that disables fsync only for the WAL. I don't think it would be a good idea. My WAL does use checksums and sequence numbers etc. to prevent committing wrong data.